1. Field of the Invention
The present invention relates generally to a micro gas turbine engine, and more specifically to an air cooled IBR for a micro gas turbine engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Very small gas turbine engines (also referred to as micro gas turbine engines) are of the size of around a few hundred pounds thrust. Larger gas turbine engines of the size to power an aircraft, such as a commercial airliner or a military jet, have steadily increased the efficiency of the engine through the years since the gas turbine engine was first commercialized. The steady increase in efficiency is mainly due to improvements in cooling of the turbine airfoils such that a higher turbine inlet temperature can be used. However, for the micro turbines, the efficiency of the engine has not changed much since the earliest ones were produced.
The main reason why the efficiency of micro gas turbine engines has not changed much is due to the turbine vanes and blades not being cooled. These engines are so small that the size of the rotor blades or stator vanes is too small to form internal cooling air passages. In the larger engines, the cooled airfoils are produced using an investment casting process also known as the lost wax process in which a ceramic core representing the cooling passages is cast into the metal airfoil and then leached away to leave the internal cooling passages within the metal airfoil.
Larger engines have individual blades secured to a rotor disk through a fir tree configuration. A slight gap exists between the blade root and the slot in the rotor disk in which compressed air can pass from the higher pressure forward side to the lower pressure aft side of the rotor disk. Cover plates are used to cover over these gaps and limit this air leakage and to protect the rotor disk sides from the high thermal temperatures from exposure to the hot gas stream.
For the micro sized gas turbine engines, using individual rotor blades attached to the rotor disk is not feasible because of the small size. The blade root to rotor disk slot gaps would be proportionally larger compared to the larger engines and therefore the leakage flow from one side of the rotor disk to the other would be very high in relation to the overall air flow. For this reason, the turbine rotor disks in the micro engines are formed as a single piece which is referred to as an IBR or integrally bladed rotor disk or blisk. The IBR is typically machined from a stock piece of material without any gaps between the blades and the disk and therefore no leakage flow is formed across the rotor disk. In the smallest of the micro turbine engines, the rotor blades are the size of a human fingernail. Thus, in order to improve the efficiency of a micro gas turbine engine, some sort of blade cooling is required.